The GnuTLS library is initialized on load; prior to 3.3.0 was initialized by calling gnutls_global_init18. gnutls_global_init in versions after 3.3.0 is thread-safe (see Thread safety).
The initialization typically enables CPU-specific acceleration, performs any required precalculations needed, opens any required system devices (e.g., /dev/urandom on Linux) and initializes subsystems that could be used later.
The resources allocated by the initialization process will be released on library deinitialization.
Note that on certain systems file descriptors may be kept open by GnuTLS (e.g. /dev/urandom) on library load. Applications closing all unknown file descriptors must immediately call gnutls_global_init, after that, to ensure they don’t disrupt GnuTLS’ operation.
The original behavior of requiring explicit initialization can obtained by setting the GNUTLS_NO_EXPLICIT_INIT environment variable to 1, or by using the macro GNUTLS_SKIP_GLOBAL_INIT in a global section of your program –the latter works in systems with support for weak symbols only.